I teach a circuit training class 2X a week at a corporate client. I've been working with the same core group of girls for about a year and a half. I've been talking to them about lifting heavy, pushing yourself harder. On Wednesday the girls had 30 seconds per exercise and my suggestion is that since its a short period of time perhaps we try to lift heavier. So as the girls moved through their stations they would comment on how heavy the last person lifted - wow, she did 80 pounds on the leg press! This one did 60 pounds on the lat pull down!